The invention relates generally to a nanostructured ferritic alloy. More particularly the invention relates to system and method of forming a nanostructured ferritic alloy having low impurities.
Gas turbines operate in extreme environments, exposing the turbine components, especially those in the turbine hot section, to high operating temperatures and stresses. In order for the turbine components to endure these conditions, they are manufactured from a material capable of withstanding these severe conditions. As material limits are reached, one of two approaches is conventionally used in order to maintain the mechanical integrity of hot section components. In one approach, cooling air is used to reduce the part's effective temperature. In a second approach, the component size is increased to reduce the stresses. However, these approaches can reduce the efficiency of the turbine and increase the cost.
In certain applications, super alloys have been used in these demanding applications because they maintain their strength at up to 90% of their melting temperature and have excellent environmental resistance. Nickel-based super alloys, in particular, have been used extensively throughout gas turbine engines, e.g., in turbine blade, nozzle, wheel, spacer, disk, spool, blisk, and shroud applications. In some lower temperature and stress applications, steels may be used for turbine components. However, conventional steels generally do not meet all of the mechanical property requirements for high temperature and high stress applications. Designs for improved gas turbine performance require alloys that balance cost with higher temperature capability.
Nickel-based super alloys used in heavy-duty turbine components require specific elaborate processing steps in order to achieve the desired mechanical properties, including three melting operations: vacuum induction melting (VIM), electro slag remelting (ESR), and vacuum arc remelting (VAR). Nano structured ferritic alloys (NFAs) are an emerging class of alloys that exhibit exceptional high temperature properties, thought to be derived from nanometer-sized oxide clusters that are precipitated in the alloys. These oxide clusters are present at high temperatures, providing a strong and stable microstructure during service. Unlike many nickel-based super alloys, which require a cast and wrought (C&W) process to be followed to obtain necessary properties, NFAs are manufactured via a different processing route that requires fewer melting steps, but includes hot consolidation following a mechanical alloying step.
Mechanical alloying requires the use of powder metal and milling media to enhance the transfer of kinetic energy to the powder metal. During mechanical alloying, impurities including, but not limited to carbon, oxygen, nitrogen, argon and hydrogen can be absorbed into the alloy, leading to detrimental second phases and/or thermally induced porosity for example. Hence, there is a need to limit and reduce the impurity phases that are introduced into the NFAs during manufacturing.